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Look into Image differentiation. It involves using a base image, say for example, your picture with 3 bottles. You can compare it with the test image (picture with 4 bottles) and then compare for differences.

Here is a link that should make things clearer -> http://docs.opencv.org/trunk/doc/tutorials/video/background_subtraction/background_subtraction.html#background-subtraction

Hope this helps!

Look into Image differentiation. It involves using a base image, say for example, your picture with 3 bottles. You can compare it with the test image (picture with 4 bottles) and then compare them for differences.

Here is a link that should make things clearer -> http://docs.opencv.org/trunk/doc/tutorials/video/background_subtraction/background_subtraction.html#background-subtraction

Hope this helps!